Infrared sensors are used in a wide range of applications, for example, for contact-less measurement of the temperature of thermal sources, such as in remote monitoring of high-temperature industrial processes or in thermal insulation assessment of buildings, or for presence detection of humans. For example, typical application areas of infrared sensors may be found in security, e.g. intruder detection, or in automotive industry, e.g. for monitoring the driver and/or passengers of a vehicle. In many applications, the cost and complexity of the infrared sensor may be an important constraint. Thermopile sensors may be considered as cheap and robust detection means for infrared sensing, which, unlike for example direct photodetection in the infrared range, can operate in room temperature conditions, e.g. without requiring active cooling.
Integrated infrared sensor devices may typically comprise at least one infrared sensing element. For example, in known sensor devices, the infrared sensor device may comprise a volume of infrared absorbing material arranged on the semiconductor wafer such that it can be exposed to incident infrared radiation. A temperature difference between this absorber and the surrounding material, which forms or thermally connects to a heat sink, may be used to generate an appropriate signal indicative of the incident radiation, e.g. a voltage, current, resistance or other electrical quantity which is influenced by the temperature difference.
The temperature difference may for example be detected by one or more thermocouples having hot junctions in or near the absorber and cold junctions in or near the surrounding material. A thermistor element may further be provided to determine a reference temperature of the surrounding material.
Good thermal isolation between the absorber and the surrounding material, such that an optimal thermal flow is channeled through the at least one thermocouple, is known to be advantageous. Therefore, the absorbing material may be arranged such that contact with the wafer material is minimized, for example, by arranging the absorber on a thin support structure, e.g. on a membrane supported by the wafer or on a bridge structure formed by removing a portion of the wafer by etching. Furthermore, the absorber may be encapsulated in a cavity, e.g. a gas-filled cavity, such as a cavity filled with air, or a vacuum cavity. Manufacturing thin support structures, such as a membrane or bridge structure, may require advanced processing techniques which are known in the art for applications in micro-electromechanical systems (MEMS).
Integrated infrared sensor devices, as known in the art, may also comprise processing means for converting the signal produced by the infrared sensing element or elements into a convenient output signal. This processing means, e.g. an application-specific integrated circuit (ASIC), may apply amplification, signal filtering, analog-to-digital conversion, and/or reference temperature correction, or may apply other processing known for this purpose in the art.
In integrated infrared sensor devices known in the art, the processing means, e.g. ASIC, may be implemented on a separate semiconductor die, and the infrared sensing element and ASIC may be operably connected by wiring and mounted in an enclosure, e.g. a transistor outline (TO) package. The top surface of such TO package may be provided with an infrared window, e.g. a filter which blocks or significantly attenuates electromagnetic wavelengths outside the infrared range, while substantially transmitting the incident infrared radiation.
However, in many applications it is desirable to obtain an integrated infrared sensor device in a compact enclosure, e.g. in a chip-level package or wafer-level package. It may be particularly advantageous to provide an integrated infrared sensor device with a reduced height profile, for example, such that the device when mounted on a carrier, e.g. on a printed circuit board (PCB), extends less than 2 mm above the mounting surface of the carrier, for example extends 1 mm above this surface or even less than 1 mm. For example, a package may need to remain lower than 1 mm above the mounting surface when mounted in order to be compatible with typical integration techniques used in the art.
In United States patent application No. US 2011/0147869, an infrared sensor is disclosed which is integrated into a wafer-scale chip package. Such package has the advantage that a larger enclosure having an infrared window provided therein, e.g. a TO package in particular, is not required. A cavity is provided below the radiation sensitive element, e.g. by etching a thin layer of epitaxial silicon, in order to thermally insulate this radiation sensitive element from the substrate. The back side of the sensor chip can be used, in accordance with this prior disclosure, as a window for infrared radiation which functions as a visible light filter. An optical element, such as a Fresnel lens, can also be formed directly on the back surface of the chip, for example by conventional photolithography processing. This sensor device further comprises bonding pads to connect the sensor device to mounting conductors through bumps, such that the device can be mounted in a low height profile arrangement on a carrier.
U.S. Pat. No. 7,180,064 discloses another related sensor device. In this device, a cavity is etched in a silicon substrate. A membrane is arranged over this cavity which comprises the radiation sensitive elements. Additional layers of absorbing or reflecting material may be provided on the membrane. The backside surface of the substrate is further provided with an antireflective coating, and an entry window is formed in an outer coating on the backside surface.